JPH05341293A - Liquid crystal display element - Google Patents

Abstract

PURPOSE:To improve a voltage holding rate and to stably obtain a display effect good over a long period of time by forming horizontally oriented films of a polyimide compd. and setting an imidization rate at a specific value range. CONSTITUTION:Transparent electrodes 5, 6 and the horizontally oriented films 7, 8 for horizontally orienting liquid crystal molecules parallel with a pair of transparent substrates 1, 2 facing each other by holding a liquid crystal material 4 therebetween are formed on the surfaces facing each other of the above- mentioned substrates. The horizontally oriented films 7, 8 consist of the polyimide compd. formed by imidizing polydiamicle and are formed by setting the imidization rate alpha at the time of imidization at 70%<=alpha<90%. Namely, the imidization rate is suppressed to 70 to 90% and, therefore, the surface energy of the surface of the oriented film decreases correspondingly. The attraction of ionic impurities in the production process is substantially prevented. Consequently, the voltage holding rate is improved and the high display grade free from flickering is obtd. in the case of the TN-LCD.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device having a polyimide horizontal alignment film.

[0002]

2. Description of the Related Art Generally, in a liquid crystal display device, a transparent electrode is attached to each of the inner surfaces of a pair of transparent substrates which face each other with a liquid crystal substance interposed therebetween, and an alignment film is further laminated thereon. There is. The alignment film regulates the alignment of the liquid crystal molecules so that their long axis directions are aligned in a predetermined direction. In a conventional TN (twisted nematic) mode or STN (super twisted nematic) mode liquid crystal display device, an alignment film made of a polyimide compound is often used as an alignment film. The polyimide alignment film is obtained by imidizing a polydiamide compound, but the polyimide alignment film obtained usually has almost completed imidization reaction. That is, the generation ratio of imide groups (hereinafter referred to as imidization ratio) is about 100%.

[0003]

However, a TFT type TN-LCD or STN-L using the above-mentioned polyimide alignment film is used.
Each CD has its own problems. That is, the TFT type TN-LCD has a low voltage holding ratio and is likely to cause flickering (flicker), and the STN-LCD has a display unevenness when operated for a long time, resulting in reduced reliability. Further, in both cases, the frequency dependence of the threshold voltage is relatively large, so that it is difficult to perform correct gradation display. So, this invention
An object of the present invention is to provide a highly reliable liquid crystal display element capable of stably obtaining accurate gradation display and high display quality.

[0004]

In order to achieve the above-mentioned object, a liquid crystal display device of the present invention has a transparent electrode and liquid crystal molecules on a surface of a pair of transparent substrates facing each other with a liquid crystal substance therebetween. A horizontal alignment film for aligning in parallel to the transparent substrate is formed, and the horizontal alignment film is composed of a polyimide compound obtained by imidizing polydiamide, and has an imidization rate α at the time of imidizing. The feature is that 70% ≦ α <90% is set.

[0005]

In the liquid crystal display device of the present invention, the imidization ratio of the polyimide alignment film is suppressed to 70% to 90%, so that the surface energy of the alignment film surface is correspondingly reduced, and ionic impurities are produced in the manufacturing process. Becomes difficult to adsorb. As a result, in the case of TN-LCD, the voltage holding ratio is improved and high display quality without flickering can be obtained. Further, in the case of STN-LCD, occurrence of display unevenness over time is suppressed and reliability is improved. Furthermore, the frequency dependence of the threshold voltage is reduced,
A liquid crystal display device capable of stably performing accurate gradation display is obtained.

[0006]

EXAMPLES The present invention will be specifically described below based on examples. FIG. 1 is a sectional view showing a typical structure of a liquid crystal display element. In the figure, a pair of transparent substrates 1 and 2 made of glass or the like is bonded via a frame-shaped sealing material 3, and a liquid crystal substance 4 is placed in a region surrounded by the sealing material 3 between the substrates 1 and 2. It is enclosed. Transparent electrodes 5 and 6 are formed on the surfaces of the substrates 1 and 2 that face each other. Then, horizontal alignment films 7 and 8 for orienting the molecules of the liquid crystal substance 4 parallel to the substrate are laminated on the transparent electrodes 5 and 6, respectively.

The horizontal alignment films 7 and 8 are each made of a polyimide compound obtained by cycloimidizing polydiamide. Examples of the polydiamide include polyamic acid, which is synthesized from tetracarboxylic acid and diamine. A polyimide compound is produced by cyclizing such a polydiamide by heat treatment or chemical treatment.

Conventionally, a horizontal alignment film made of a polyimide compound is formed by advancing the imidization ratio to 90 to 100% as described above in order to obtain a sufficient alignment effect. In such a highly oriented imidized polyimide alignment film, the surface of the alignment film is active, so that ionic impurities are easily adsorbed in the manufacturing process. Therefore, in the TN-LCD, the ionic impurities adsorbed on the horizontal alignment film are dissolved into the liquid crystal substance and the voltage holding ratio of the voltage applied between the transparent electrodes (driving voltage of the liquid crystal display element) is lowered. , Flickering becomes noticeable on the display screen. Also, conventional STN-LC
In D, when operating for a long time, ions in the liquid crystal substance are adsorbed on the surface of the alignment film, display unevenness occurs, and the reliability decreases. Furthermore, there is a drawback that the frequency dependence of the threshold voltage becomes large and it is difficult to perform accurate gradation display, which is common to the conventional liquid crystal display elements using a polyimide alignment film in which imidization has advanced to a high degree.

In contrast to the conventional liquid crystal display element as described above, the liquid crystal display element of this embodiment controls the imidization ratio to about 80%, so that the surface energy becomes small and ionic impurities are adsorbed. It becomes difficult, and the voltage holding ratio becomes high in the TN-LCD. Further, in the polyimide alignment film, when the imidization ratio is lowered, the polar force component of the surface energy composed of the dispersive force component and the polar force component becomes small, and this polar force component locally adsorbs mobile ions in the liquid crystal substance. Has a large impact on Therefore, in this embodiment, the horizontal alignment films 7 and 8 are used.
By controlling the imidization ratio of the polyimide compound that forms the compound within a proper range, it is possible to optimize the polar component while ensuring the necessary alignment effect on the liquid crystal molecules, and improve the reliability of the liquid crystal display element. Further, by reducing the frequency dependence of these liquid crystal display elements, accurate gradation display can be performed. In this way, the liquid crystal display element of this embodiment can solve the above-mentioned various problems and stably obtain good display quality as a liquid crystal display element. The imidization ratio of the above-mentioned polyimide compound can be controlled by a method such as adjusting the temperature at the time of imidization by heat treatment. The imidization ratio can be grasped by detecting the imidized group in the amide groups of the whole polydiamide by a known analysis method.

Table 1 shows a total of 80% and 100% of the imidization ratios of polyimide alignment films for three types of STN-LCDs (LC-1, LC-2, LC-3) using different liquid crystal substances. 6
Each voltage holding ratio of the sample is shown.

[0011]

[Table 1]

According to Table 1, in any STN-LCD, the voltage holding ratio is higher when the imidization ratio is 80% than when it is 100%. From this, it is understood that a polyimide alignment film having an imidization ratio of 80% can stably obtain higher display quality than a polyimide alignment film having an imidization ratio of 100%.

Table 2 also shows three types of STN-LCD (L
C-1, LC-2, LC-3), the voltage of the applied voltage is 70 Hz, 1 kHz, 35 kHz for the six liquid crystal display elements with the polyimide orientation film having an imidization ratio of 80% and 100%, respectively.
The result of measuring each threshold voltage in the case of driving by switching to three stages is shown. In Table 2, each threshold voltage is shown as a ratio when the threshold voltage is "1" when a pulse voltage having a frequency of 1 kHz is applied.

[0014]

[Table 2]

From Table 2, in any of the STN-LCDs, the threshold voltage with respect to the frequency change is higher when the polyimide alignment film having an imidization ratio of 80% is used than when the polyimide alignment film having an imidization ratio of 100% is used. It can be seen that the change in is small. Therefore, when gradation display is performed by STN-LCD by time-division driving, by using a polyimide alignment film having an imidization ratio of 80%, the imidization ratio is 10%.
More accurate gradation display can be performed as compared with the case of using 0%. Table 3 shows the results of measuring the pretilt angles when the imidization ratios of the polyimide alignment film were 80% and 100%, respectively, for the same three types of STN-LCDs.

[0016]

[Table 3]

From Table 3, the imidization ratio is 100% to 80.
It can be seen that the pretilt angle of the liquid crystal molecules is also reduced by decreasing the value to%, but the pretilt angle required for STN-LCD is sufficiently secured. Therefore, S
In the TN-LCD, even if the imidization ratio of the polyimide alignment film is suppressed to about 80%, there is no possibility of causing uneven alignment of liquid crystal molecules due to insufficient pretilt angle.

The degree of suppressing the imidization ratio of the polyimide alignment film is limited to 70%. This is because when the imidization ratio is less than 70%, the pretilt angle of each liquid crystal molecule varies due to the weak alignment regulating force with respect to the liquid crystal molecules, and the hardness of the alignment film surface is insufficient, and the alignment treatment by rubbing is performed. This is because problems such as peeling of the polyimide alignment film may occur. Therefore, it is desirable to control the imidization ratio of the polyimide horizontal alignment film in the range of 70 to 90%.

The present invention can be applied to a case where a chemical treatment method is used as a method for imidizing a polydiamide or a case where heat treatment and chemical treatment are used in combination.

[0020]

As described above in detail, according to the present invention, the horizontal alignment film is formed of a polyimide compound, and the imidization ratio of the polyimide compound is set to 70 to 90%, which is necessary. While maintaining the alignment effect, the voltage holding ratio is improved, and a good display effect without flicker or unevenness can be stably obtained for a long time. Further, the frequency dependence of the threshold voltage in the liquid crystal display element is reduced, and it is possible to accurately perform gradation display.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing a liquid crystal display element as an embodiment of the present invention.

[Explanation of symbols]

 1, 2 substrate 3 sealing material 4 liquid crystal material 5, 6 transparent electrode 7, 8 horizontal alignment film

Claims (1)

[Claims] 1. A transparent electrode and a horizontal alignment film for aligning liquid crystal molecules in parallel with the transparent substrate are formed on surfaces of the pair of transparent substrates facing each other with a liquid crystal material sandwiched therebetween. The liquid crystal display device is characterized in that the horizontal alignment film is made of a polyimide compound obtained by imidizing polydiamide, and the imidization ratio α at the time of imidization is set to 70% ≦ α <90%.